Memory & Storage: Different Tasks, Different Technologies

Computers have employed various technologies to preserve information. Most fall into two broad categories: memory and storage.

Memory holds running programs and information the processor is currently using. Storage preserves data and programs for future use.

Why treat them differently? Memory must be fast and flexible. Storage has to be big, permanent, and affordable. No single technology meets all those requirements.

Pretending You Have More Memory Than You Do

Abundant memory lets computers juggle many tasks simultaneously and swiftly. But memory is comparatively expensive. In the 1950s, engineers developed a way to “fake it.”

Virtual memory (paging systems) uses disk storage as an extension of memory, letting even small computers run big programs. All modern operating systems use virtual memory.

Tech Talk: How Does Virtual Memory Work?

Main memory is expensive. And like most expensive things, there’s never enough. “Virtual memory,” first prototyped in 1959 for the University of Manchester’s Atlas computer, uses drum or disk memory to simulate a larger main memory. How does it work?

Imagine a large “virtual” memory divided into big (1,024 word) blocks called pages. Only some pages can fit in main memory. The rest are temporarily stored on the disk. The computer hardware knows which pages are where.

If the program references a page in main memory, it uses it immediately. But when the program references a page not in memory, the operating system interrupts and reads the missing page from the disk. If the page displaced by the new addition has changed, it is written to the disk. The program then resumes and accesses the new page as if nothing had happened—except a time delay.